[1.7.x] Fixed flake8 warnings introduced in recent commits.

Backport of 79f05616fbf48cf7c205ef17666af0c3d47b3c1e from master

django/db/backends/postgresql_psycopg2/base.py

@@ -4,8 +4,6 @@ PostgreSQL database backend for Django.
 Requires psycopg 2: http://initd.org/projects/psycopg2
 """
 
-import sys
-
 from django.conf import settings
 from django.db.backends import (BaseDatabaseFeatures, BaseDatabaseWrapper,
     BaseDatabaseValidation)

tests/prefetch_related/tests.py

@@ -1066,7 +1066,7 @@ class Ticket21760Tests(TestCase):
         for _ in range(3):
             house = House.objects.create()
             for _ in range(3):
-                self.rooms.append(Room.objects.create(house = house))
+                self.rooms.append(Room.objects.create(house=house))
 
     #@override_settings(DEBUG=True)
     def test_bug(self):

tests/test_discovery_sample/doctests.py

@@ -3,6 +3,7 @@ Doctest example from the official Python documentation.
 https://docs.python.org/3/library/doctest.html
 """
 
+
 def factorial(n):
     """Return the factorial of n, an exact integer >= 0.
 
@@ -35,7 +36,7 @@ def factorial(n):
         raise ValueError("n must be >= 0")
     if math.floor(n) != n:
         raise ValueError("n must be exact integer")
-    if n+1 == n:  # catch a value like 1e300
+    if n + 1 == n:  # catch a value like 1e300
         raise OverflowError("n too large")
     result = 1
     factor = 2

tests/test_runner/test_discover_runner.py

@@ -105,7 +105,6 @@ class DiscoverRunnerTest(TestCase):
 
     def test_testcase_ordering(self):
         suite = DiscoverRunner().build_suite(["test_discovery_sample/"])
-        tc_names = [case.__class__.__name__ for case in suite._tests]
         self.assertEqual(
             suite._tests[0].__class__.__name__,
             'TestDjangoTestCase',